Aqueous solutions of ammonium fluoride are produced as by-products in chemical processes involving the replacement of fluoride by ammonia, such as in the production of 4-amino-3,5-dichloro-2,6-difluoropyridine from 3,5-dichloro-2,4,6-trifluoropyridine, and in many other processes. Aqueous ammonium fluoride solutions are also used in frosting glass and in metal pickling processes. Disposal of these solutions as waste is difficult since fluoride ion is considered to be a hazardous waste and disposal further results in the loss of valuable raw materials. These costs and environmental factors make it important to find ways to recover both the ammonia and the fluoride values from aqueous mixtures containing ammonium fluoride.
Ammonia is conventionally recovered from many ammonium salt solutions by basifying and subsequently distilling to remove the liberated ammonia.
Previously utilized methods of removing fluoride ion from dilute aqueous solutions include precipitation as calcium fluoride, collection in an ion exchange medium, and distillation as hydrogen fluoride. The fluoride containing mixtures obtained are generally discarded.
Ammonium fluoride solutions prepared by the action of aqueous ammonia on fluorosilicic acid have been utilized in the commercial production of calcium fluoride by precipitation with lime. The ammonia is recycled in the conventional way in this process. Additionally, aqueous ammonium fluoride solutions have been dehydrated and the resulting dry crystals pyrolyzed to produce ammonium bifluoride. In other operations, the ammonia and fluoride values of aqueous ammonium fluoride solutions have been recovered by evaporation and subsequent treatment with sulfuric acid at high temperatures to obtain hydrogen fluoride as a distillate and ammonium bisulfate or, after ammonation, ammonium sulfate as a solid residue. This process is known to involve large evaporation and filtration costs and to require a nearby market for ammonium sulfate.
It is, therefore, of economic importance to discover improved methods of recovering in useful forms the ammonia and fluoride ion values present in aqueous ammonium fluoride solutions.